How to test main class of Spring-boot application

Question

I have a spring-boot application where my @SpringBootApplication starter class looks like a standard one. So I created many tests for all my functi

Answer 1

In addition to the answers above, here is a unit test of a SpringBoot application's main method for if you are using JUnit 5:

try (MockedStatic<SpringApplication> mocked = mockStatic(SpringApplication.class)) {
            
   mocked.when(() -> { SpringApplication.run(ElectronicGiftCardServiceApplication.class, 
      new String[] { "foo", "bar" }); })
         .thenReturn(Mockito.mock(ConfigurableApplicationContext.class));
            
   ElectronicGiftCardServiceApplication.main(new String[] { "foo", "bar" });
            
   mocked.verify(() -> { SpringApplication.run(ElectronicGiftCardServiceApplication.class, 
      new String[] { "foo", "bar" }); });

}   

It verifies that the static method run() on the SpringApplication class is called with the expected String array when we call ElectronicGiftCardServiceApplication.main().

Same idea as awgtek and Ramji Sridaran, but their solutions are for JUnit 4.

Answer 2

I had the same goal (having a test that runs the main() method) and I noticed that simply adding a test method like @fg78nc said will in fact "start" the application twice : once by spring boot test framework, once via the explicit invocation of mainApp.main(new String[] {}), which I don't find elegant.

I ended up writing two test classes : one with @SpringBootTest annotation and the empty test method applicationContextLoaded(), another one without @SpringBootTest (only RunWith(SpringRunner.class)) that calls the main method.

SpringBootApplicationTest

package example;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.boot.test.context.SpringBootTest;

@RunWith(SpringRunner.class)
@SpringBootTest
public class SpringBootApplicationTest {

  @Test
  public void contextLoads() {
  }
}

ApplicationStartTest

package example;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.test.context.junit4.SpringRunner;

@RunWith(SpringRunner.class)
public class ApplicationStartTest {
  @Test
  public void applicationStarts() {
    ExampleApplication.main(new String[] {});
  }
}

Overall, the application is still started two times, but because there is now two test classes. Of course, with only these two tests methods, it seems overkill, but usually more tests will be added to the class SpringBootApplicationTest taking advantage of @SpringBootTest setup.

Answer 3

This simple mock test for SpringApplication does not invoke any methods but just tests the starter app. [uses PowerMockRunner.class]

import org.junit.Test;
import org.junit.runner.RunWith;
import org.powermock.api.mockito.PowerMockito;
import org.powermock.core.classloader.annotations.PowerMockIgnore;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import org.springframework.boot.SpringApplication;

@RunWith(PowerMockRunner.class)
@PowerMockIgnore({"com.sun.org.apache.xerces.*", "javax.xml.*", "org.xml.*", "javax.management.*"})
public class JobsAppStarterTest {

    @Test
    @PrepareForTest(SpringApplication.class)
    public void testSpringStartUp() {
        PowerMockito.mockStatic(SpringApplication.class);
        SpringApplication.run(JobsAppStarter.class, new String[] {"args"});
        JobsAppStarter.main(new String[] {"args"});
    }
}